The maintenance of the proper amount of humidity in the air is important for the well being of humans. For example, many people recommend that the humidity level in the living environment be maintained at between 40% and 50% at normal room temperature (approximately 72 degrees F).
Because the total humidity which the air may contain is in relation to the temperature of the air, the proper humidity level is easily maintained and very often largely surpassed in the summer while it can often be maintained in the winter time only by adding humidity to the atmosphere in the home. Indeed, it is necessary to replace the humidity which is lost when warm "humid" inside air is condensed in or on external windows, walls or ceilings or is expelled outside through doors, windows or other openings and such air is replaced by cold air containing much less humidity.
Taking a hot shower or letting the kettle boil will add humidity to the air. However, such additional humidity is usually insufficient to maintain a proper humidity level in the home.